Multiple sclerosis (MS) relapses are known to be significantly decreased during pregnancy. This proposal will establish whether oral treatment with estriol, the major estrogen of pregnancy, induces a decrease in relapses in relapsing remitting multiple sclerosis (RRMS) subjects when used in combination with injectable Copaxone. Previously, in a pilot study, it has been demonstrated that treatment of RRMS subjects with oral estriol for six months resulted in a significant reduction in gadolinium enhancing lesions on serial brain MRIs (Annals of Neurology, 2002;52:421-428) and caused a favorable shift in immune responses (Journal of Immunology, 2003;171:6267-6274). This is an add-on study aiming to extend these previous findings by treating longer and focusing on clinical outcomes. The combination of Copaxone injection plus estriol pill (8 mg per day) will be compared to Copaxone injection plus placebo pill in a double blind trial. The duration of treatment will be two years and the primary outcome measure will be relapse rate. Secondary outcomes will include disability measures (MSFC, EDSS, 7-24 MS Quality of Life, Modified Fatigue Impact Scale and Beck Depression Inventory) and MRI surrogate biomarkers for relapses (enhancing lesions, new T2 lesions) and disability (atrophy, T1 holes). Safety measures (blood tests and gynecologic evaluations) will also be followed. The overall goal of this study will be the development of an oral treatment, estriol, for RRMS.